The present invention relates to a method and a system for sampling cryogenic liquids and to an air separation unit provided with at least one such system.
Such cryogenic liquids, which are usually oxygen, nitrogen or argon, are at a temperature below about −170° C. They are in particular produced by the use of a distillation column forming part of an air separation unit.
It is known to take samples of these cryogenic liquids for the purpose of subsequently analysing them. This then makes it possible to check, in particular, the content of low-volatility impurities in these liquids, such as nitrous oxide N2O, carbon dioxide CO2 or hydrocarbons CnHm.
This is because, at cryogenic temperatures, some of these impurities are liable to be deposited in the components of the air separation units, in particular in the reboiler/condenser of the distillation columns. It is therefore essential to monitor the impurity content, both in terms of product quality and plant safety.
When it is a question of analysing impurities of low volatility, there is the difficulty of obtaining a vaporized sample that is as representative as possible of the liquid to be analysed.
This is because the analytical methods commonly used, such as gas chromatography or infrared spectroscopy, involve warming the specimen taken up to a temperature close to room temperature. For this purpose, it is first necessary to vaporize the cryogenic liquid sampled, and then warm it.
Under these conditions, to obtain an analysis representative of a bath of cryogenic liquid, it is necessary, on the one hand, to take from it a liquid sample representative of the average composition of the entire bath, and then to vaporize it rapidly and completely. In the case of the air separation unit, two cryogenic liquid sampling modes are known in particular.
The first of these, also called “liquid lift”, is based on the thermosyphon effect. To achieve this, a by-pass is made for the liquid to be analysed, in which by-pass the flow is provided by the vaporization of a fraction of this liquid.
This liquid lift is diverted to the wall of the cold box of the air separation unit, within an insulated container, for example insulated by rockwool, so as to limit any heat influx. A continuous sample of the cryogenic liquid flowing in this lift is then vaporized in a finned atmospheric heat exchanger, associated with a mixer, which is commonly called “flash vaporization”.
An alternative mode of sampling, also called capillary sampling, consists in withdrawing the liquid under pressure through a capillary, namely a first tube of small inside diameter, for example about 0.5 mm. This tube is then taken, in a second tube, of larger cross section, to a hot spot where all of the liquid to be analysed instantly vaporizes.
These known sampling systems are widespread and guarantee results that are generally satisfactory. However, they do have a number of drawbacks.
Thus, they may introduce a problem as to the representativeness of the sample taken, in particular as regards capillary sampling, since the capillary, if it is connected to a liquid bath, does not allow forced flow of the said liquid to be analysed.
Moreover, these systems are subject to ageing, particularly in the case of the liquid lift.
This is because, in the latter case, there is progressive ingress of moisture into the insulation chamber, causing the formation and then the build-up of ice. The heat influx then becomes such that the liquid flow may be affected thereby.